I say "if" because, while it works for your simple test case, I can't guarantee this trick will help with your real-world problem. RightToLeft mode is one of .NET's more innovative features--offhand, I can't think of another flavor that has anything equivalent to it. The official documentation on it is sparse (to put it mildly), and so far there don't seem to be a lot developers using it and sharing their experiences online. So try it and see what happens.

If you want to match only "ab" you can use ab.? or a.{1} (or a lot of other options)
If you want to match only "abc" you can use ab. or a.{2} (or a lot of other options)
If you want to match only "abcd" you can use ab.* or a.{3} (or a lot of other options)